Noise emissions from turbine engines, particularly those used to power aircraft, are an important environmental concern. Many industrialized countries already impose restrictions on turbine engine noise emissions, and it appears likely that noise restricting regulations will become increasingly stringent and more geographically widespread in future years. Recent generation engines incorporate advanced noise suppression technologies to ensure that their noise emissions are within regulatory limits. Older generation engines, however, were designed in a more permissive era and it has been necessary to acoustically upgrade these engines to bring them into regulatory compliance. However the acoustic upgrades are expensive and almost invariably diminish engine efficiency. Moreover, as noise regulations become increasingly strict, it can become inordinately difficult and prohibitively expensive to introduce further acoustic upgrades into older generation engines. As a result, many of these engines may have to be retired from service despite being serviceable in all other respects.
One type of older generation turbine engine that remains popular and widely used even today, is a mixed flow turbofan engine. Acoustically upgraded versions of these engines have an exhaust system that includes a convoluted or lobed mixer. The mixer is circumscribed by an engine case that extends longitudinally aftwardly of the mixer. In operation, the mixer causes an inner stream of high temperature, high velocity core gases to mix with an outer, concentric stream of relatively cool, lower velocity air. The partially mixed gas stream, which has an average velocity intermediate that of the inner and outer streams, is then expelled through an exhaust nozzle. Since exhaust noise emissions are proportional to the eighth power of velocity, the lobed mixer's ability to decelerate the core gases by mixing those gases with the lower velocity concentric air stream makes the lobed mixer highly effective as an exhaust noise suppressor.
As increasingly strict noise regulations have been imposed, various improvements to the basic lobed mixer have been devised. Among these improvements are mixers having an increased quantity of lobes, and the unique double lobed mixer described in U.S. Pat. No. 5,638,675 to Zysman et al., and commonly assigned to the assignee of the present application. However efficacious these improved mixers may be, they are not without their shortcomings. Their larger surface area, in comparison to that of the basic lobed mixer, results in increased skin friction and aerodynamic drag, leading to diminished engine efficiency and degraded aircraft performance. The geometric complexity of the improved mixers also escalates the costs associated with their manufacture. Moreover, engines that have been fitted with the basic lobed mixer can only benefit from the improvements if the basic mixer is replaced with one of the improved mixers--an expensive and therefore unappealing option.
A further potential drawback of lobed mixers arises from fluid shearing that occurs at the juncture of the high and low velocity streams. While the admixture of the fluid streams diminishes acoustic energy amplitudes at the low frequency end of the noise spectrum, the fluid shearing can increase the energy amplitudes at higher frequencies. As a rule, higher frequency spectral components propagate less readily than lower frequency components, but they are also more annoying and therefore are particularly undesirable. The U.S. Government's formula for quantifying turbine engine noise emissions penalizes the presence of the higher frequency components by assigning high weighting factors to those components. Existing lobed mixers (including the basic lobed mixer and the improved versions described above) are carefully designed to capture the benefits of low frequency noise reduction while not amplifying the higher frequencies enough to exceed the regulatory threshold. Nevertheless, any modification of an existing mixer with intent to improve its acoustic characteristics carries the risk of upsetting this carefully crafted balance.